The invention relates to a process and an apparatus for applying an optical coating to a front surface of a substrate located in a vacuum chamber.
A range of vacuum coating processes are used to produce optical coatings, for example for antireflection and mirror coatings, optical filters and other treatments of substrate surfaces. The vacuum coating processes carried out in a vacuum chamber include, inter alia: vapor deposition (VD), chemical vapor deposition (CVD) and sputtering (physical vapor deposition—PVD).
During vapor deposition, first of all a coating material is melted and vaporized by means of a heater or electron beam bombardment. The material in the vapor phase then condenses on a cooler surface of the substrate which is to be coated, where it forms a layer. However, deposits are also formed on other surfaces of the vacuum chamber which are accessible to the vapor phase.
Unlike in the case of physical vapor deposition processes, in the CVD process a solid component which is initially in a vapor phase is deposited on the surface of the substrate as a result of a chemical reaction. The precondition for this is for a compound having the desired properties to exist in a vapor phase which under defined conditions, for example with a defined reaction partner, is deposited as a solid layer. One specific type of the CVD process is plasma-enhanced CVD. In this variant, a plasma is generated above the substrate surface to be coated. Some of the components within the vapor which is introduced are brought into a chemically reactive state in the plasma, so that they are capable of undergoing a chemical reaction in the vicinity of the substrate surface, which leads to a solid layer building up.
The term sputtering is used to describe removal of material by ion bombardment, during which extremely small particles of a bombarded solid-state target are detached (sputtered) and, if appropriate after reaction with sputtering gases present in the vacuum chamber, condense on a front surface located directly opposite the target, of a substrate, resulting in the formation of a solid layer. Sputtering is a predominantly physical coating process with a directional characteristic caused by the ion bombardment, in which coating is carried out only in regions which directly face the target, in particular the front surface of the substrate.
The above mentioned coating processes are typically used to apply layers of inorganic materials, which often have ceramic properties. However, inorganic layers have the drawback of generally being very brittle, which means that they are of only limited suitability, for example, for application to plastic substrates with different mechanical and/or thermal properties, in particular an elasticity and expansion coefficient which are different from those of the inorganic layer. This plays a role in particular for plastic spectacle lenses or watch faces, where antireflection coatings and/or scratchproof layers are exposed to high mechanical and thermal stresses and have to satisfy high demands in terms of the optical quality. Major differences in the mechanical and/or thermal properties of the substrate and functional layer lead to reduced adhesion and possibly even to the layers flaking off. Layers flaking off spectacle lenses, for example, may lead to vision being impaired when wearing the spectacles.
To solve this problem, it is known to seal plastic surfaces by means of what are known as hard lacquers by wet-chemical processes, so as to harden them. After the surface has been conditioned in this way, a further optical layer or a layer system is then applied to the hard lacquer layer, for example by one of the vacuum coating processes mentioned above. However, these hard lacquer coating operations are very technically complex, produce only a low yield of optically perfect hard lacquer layers and have a relatively low resistance to abrasion and weathering compared to the inorganic layers applied by means of vacuum coating processes, such as oxide or nitride layers. Furthermore, a two-stage production process of this type entails increased costs on account of possible surface contamination and more complex handling techniques.
An alternative solution approach, which makes do without a separately applied hard lacquer layer, is known from EP-A-0870070 and corresponding U.S. Pat. No. 6,130,002. In this case, in a plasma-enhanced CVD process, organometal monomers in a vapor phase are introduced into the vacuum chamber in addition to the reactive gases oxygen and nitrogen and are incorporated into the layer as it is being built up. The organometal monomer hexamethylenedisiloxane (HMDSO) is used to produce an organically modified oxide, oxynitride or nitride layer. Preferred application areas for this process include film coatings, window and mirror coatings and coatings for decorative surfaces and exterior cladding applications.
A disadvantage which has emerged with plasma-enhanced CVD processes is that not only the desired surfaces of a substrate but also other surfaces in the vacuum chamber, in particular including rear sides of the substrates, are coated or contaminated in an uncontrolled way. This occurs at least because the process conditions are selected in such a way that the reactive gas reacts chemically at the surfaces. Accordingly, time-consuming and expensive cleaning work has to be carried out on the vacuum chamber during the production process. Since uncontrolled coating of the rear side affects the optical quality of the end product, CVD processes of this type cause problems with the production of optical elements.
The process described in EP-A-0870070 and U.S. Pat. No. 6,130,002 produces a certain localization on surfaces to be coated, by generating a high-density plasma zone immediately in front of the surface to be coated, and by targeted passage of reactive gases and monomers on to the surface to be coated. However, this process is technically complex and does not sufficiently prevent contamination of the vacuum chamber and uncontrolled coating of the rear side of the substrates.
The object of the present invention is to provide a process and an apparatus for producing coatings which allow a layer with defined mechanical properties to be applied to a front surface of a substrate with little contamination of a vacuum chamber.